The invention relates to a method for producing transparent protective coatings, especially on optical plastic substrates, by chemical vapor deposition by the action of a plasma (plasma CVD) on a polymerizable monomeric organic compound from the group of the siloxanes and silazanes, wherein oxygen is supplied in excess to the polymerization process.
DE-OS No. 34 13 019 (Federal Republic of Germany Application) discloses a method of the kind described above. This disclosure, however, is completely silent on the potential conditions on the substrates during the coating, and it is recommended, for the purpose of increasing the coating hardness, to add substances such as oxygen in the coating process which are said to increase the coating hardness. In order in this case to achieve sufficient strength of adhesion, however, it is furthermore prescribed that the substances increasing the hardness are not to be added until after the polymerization process has begun. A less than stoichiometric supply of oxygen at the beginning, however, results in brownish coatings. The object of the invention is to further improve the strength of adhesion and increase the coating hardness without complicating the performance of the process or having to accept brownish coatings.
The stated object is achieved in accordance with the invention, in the process described above, in that
(a) the plasma is produced by means of radiofrequency between two electrodes of which one acts as a cathode, PA0 (b) the substrates are disposed on the electrode acting as cathode, PA0 (c) the substrates are exposed prior to the coating, in the absence of the organic compounds, to an ion bombardment by a glow discharge in an atmosphere containing a noble gas.
The two electrodes can be formed by the reaction chamber on the one hand and a substrate holder contained therein on the other, the substrate holder being connected to a radiofrequency generator through a capacitance and an impedance matching network. On account of the surface area ratios between the necessarily smaller substrate holder and the vacuum chamber surrounding it, a negative bias establishes itself on the substrate holder, so that it is given a cathode function. By this bias, which is superimposed on the radiofrequency voltage, positive ions are pulled from the plasma and accelerated toward the cathode. In this manner, solid and dense coatings are deposited on the substrates, and as a rule they are extremely hard due to the additional ion bombardment.
In connection with this, the composition of the gases or gas mixtures fed to the coating process is very important. If the measures prescribed according to the invention are not taken, brown coatings form at coating thicknesses above about 0.5 to 2 micrometers, and these are useless for optical applications since they strongly absorb the blue and violet parts of the visible spectrum.
By the measure of exposing the substrates before they are coated to a glow discharge in an atmosphere containing noble gas, in the absence of the organic compounds, an excellent preparation of the substrate for the coating that follows is achieved and leads to a great strength of adhesion.
It has especially been found that the coatings produced in accordance with the invention have excellent adherence to the CR 39, the trademark for polydiethylene allyl diglycol carbonate, commonly used for plastic eyeglasses and they easily withstand the conventional rubber eraser test.
The transition from the preparatory step of exposing substrates before coating to the actual coating process can be accomplished simply by turning on the feed of the monomeric organic compounds from the group of the siloxanes and silazanes, and of the oxygen, while at the same time shutting off the noble gas feed.
It is especially advantageous to use as the siloxane, the compound hexamethyldisiloxane, which has a relatively high vapor pressure. In conjunction with the usual process parameters, i.e., with an excess of oxygen, it is possible to produce coatings which are completely transparent in the visible range of the spectrum and which have an extraordinarily high microhardness of HV.sub.0.01 =1000 Kp/mm.sup.2. This hardness is to be compared with that of quartz, which has a hardness of 600 to 750 Kp/mm.sup.2.
In accordance with the further invention, it is especially advantageous to set the bias voltage at levels between -20 and -500 V, preferably between -80 and -250 V, by selecting the input power.
When hexamethyldisiloxane and a bias voltage of about -200 V were used, the coating rate amounted to about 18 micrometers per hour or 5.0 nm/s. This rate is an average; higher coating rates can be achieved by increasing the pressure and/or the bias voltage.
The polymerization is preferably performed in the pressure range between 10.sup.-3 and 10 mbar.
The refractive index of the quartz-like coatings produced is in the neighborhood of that of quartz, and can be made slightly higher or lower by modifying the process parameters.